TCP/IP Concepts
Packet switching - data streams are broken down into packets, and routing devices multiplex data originating from multiple sources, onto the same lines.
Protocol layering - each protocol adds some small amount of features, and allows carrying higher-level protocols over its own packets.
Hardware independence - only the lower-level protocols (i.e. IP) have any relation to hardware, and are designed to be able to work over many types of lines and communication media.